Littrow expansion, named after the Austrian physicist Heinrich Littrow, is a method used in optics and diffraction-grating theory. It specifically pertains to the analysis of light diffraction by a grating at a specific angle, known as the Littrow angle. In the context of a diffraction grating, the Littrow expansion occurs when the incoming light is focused so that the angle of incidence equals the angle of diffraction for one of the diffracted orders.
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